Currently, there exist a variety of propulsion or drive technologies used to power vehicles. These technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of various drive systems. The increasing cost of fossil fuels and the desire to improve fuel economy and reduce emissions in vehicles have led to the development of advanced hybrid vehicles.
Hybrid vehicles typically include an internal combustion engine and an electric traction motor. Hybrid vehicles also may include two separate DC energy sources for the electric traction motor. During varying driving conditions, hybrid vehicles will alternate between these separate energy sources, depending on the most efficient manner of operation of each energy source.
Hybrid vehicles are also broadly classified into series or parallel drives, depending upon the configuration of the drivetrains. In the series drivetrain utilizing the ICE and the electric traction motor, only the electric motor drives the wheels of the vehicle. The ICE converts a fuel source into mechanical energy, which turns a generator that converts the mechanical energy into electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, the ICE and the electric traction motor operate in parallel to propel the vehicle.
Secondary/rechargeable batteries are an important component of a hybrid vehicle system. Secondary batteries store energy that is used by the electric traction motor to drive the vehicle. In addition, secondary batteries enable an electric motor/generator (MoGen) to store energy that is recovered during braking. Accordingly, the batteries perform load balancing, absorbing, or delivering the instantaneous difference in energy generated by the ICE with that required by driving conditions.
A battery module may be comprised of several series-connected electrochemical cells. Typical electrochemical cell voltages are in the one to two volt range. Present battery module output voltages are in the 12 to 42 volt range. Conventional vehicle traction drive systems operate with a DC bus voltage in the range of approximately 300 to 400 volts. In conventional electric or hybrid vehicle applications, battery modules are connected in series to provide the desired DC voltage levels required by the high voltage vehicle traction drive system. Generally speaking, a high voltage vehicle traction drive system provides cost, performance and weight advantages, as compared to low voltage traction drive systems.
Electric vehicles, including battery, hybrid, and fuel cell electric vehicles, typically use an inverter in the form of a switch-mode power supply to provide multi-phase operating power to the vehicle's electric drive motor. The inverter design most commonly used is a pulse width modulated (PWM) voltage source inverter which utilizes power transistors that can supply the high currents needed to satisfy the torque demands required by the vehicle drive motor. The inverter switches power to the drive motor windings from a direct current (DC) bus. For a low voltage system, the DC bus operates at approximately 42V for a high voltage system, the DC bus operates at approximately 350–400 volts (VDC).
The standard method to interface energy storage into an electric propulsion system for hybrid vehicles is to employ a power converter between the energy storage system and main propulsion DC bus. However, it should be appreciated that usage of such power converter unnecessarily adds to the complexity and cost of the vehicle.